Federal and local investigators searched through debris, looking for remnants of a possible explosive device, after a blast at a Southern California medical office building that left a woman dead and three other people injured.

The explosion occurred around 1 p.m. Tuesday in Aliso Viejo, about 50 miles (80.5 kilometers) south of Los Angeles. It blew siding off the walls, exposing insulation and framing and shattering windows at the two-story building that houses medical offices.

"The corner of that building, the whole bottom floor is pretty much blown out," fire Capt. Tony Bommarito said.

The cause of the blast remained a mystery for much of the day, with officials initially believing a car had smashed into the building. But sheriff's officials said the size of the blast made it suspicious.

A federal official told The Associated Press late Tuesday that the explosion was believed to be an intentional act and the blast may have come from a package.